#include <atomic>
#include <cassert>
#include <thread>

#include <glog/logging.h>

std::atomic<bool> x, y;
std::atomic<int> z;

void write_x()
{
    x.store(true, std::memory_order_seq_cst);
}

void write_y()
{
    y.store(true, std::memory_order_seq_cst);
}

void read_x_then_y()
{
    while (!x.load(std::memory_order_seq_cst))
        ;
    if (y.load(std::memory_order_seq_cst)) {
        LOG(INFO) << "z: " << z++;
    }
}

void read_y_then_x()
{
    while (!y.load(std::memory_order_seq_cst))
        ;
    if (x.load(std::memory_order_seq_cst)) {
        LOG(INFO) << "z: " << z++;
    }
}

int main(int argc, char* argv[])
{
    google::SetStderrLogging(google::LogSeverity(FLAGS_minloglevel));
    google::InitGoogleLogging(argv[0]);
    x = false;
    y = false;
    z = 0;
    std::thread a(write_x);
    std::thread b(write_y);
    std::thread c(read_x_then_y);
    std::thread d(read_y_then_x);
    a.join();
    b.join();
    c.join();
    d.join();
    LOG(INFO) << "z: " << z;
    assert(z.load() != 0); // 安全！z是1，或2
}
